The present invention relates to a planar antenna and, more particularly, to a planar antenna having plate-shaped radiators excited by narrow slots cut in a waveguide to radiate microwaves into space.
A microwave antenna using a parabolic reflector is in widespread use as a ground antenna for transmitting and receiving microwaves in satellite broadcasting. However, this antenna has a large-scaled parabolic reflector, and is easily influenced by weather conditions (e.g., snow, wind, and the like).
A planar antenna is free from the above-mentioned problems, and can be efficiently installed on the ground without requiring a large space, since it does not require any large reflector like the parabolic antenna. Therefore, the use of a planar antenna has been proposed for use as a ground antenna for transmitting and receiving microwaves in satellite broadcasting. Planar antennas include various types of antennas. For example, in a slot antenna, a plurality of slot arrays formed on the upper plate of a wide, thin substrate are excited by feed wire lines (or microstrip lines) and radiate microwaves from radiators. A planar type slot array antenna of this type is well known to the skilled in the art.
Since the planar type slot antenna has a main part constituted by a relatively thin substrate, it is not easily influenced by the weather conditions, and can be easily installed on the ground. However, the aperture efficiency of this antenna is lower than that of a parabolic antenna. The low aperture efficiency is caused by high dielectric and conductor losses since power is fed to the radiatiors through relatively long microstrip lines.
As a recent planar type slot antenna with an improved aperture efficiency, a radial slot antenna for 12-GHz satellite TV reception is described in IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION. VOL. AP-33, NO. 12, December 1985, pp. 1347-1353. With this slot antenna, since a circular waveguide is used in place of wire lines for feeding power to radiators, the dielectric and conductor losses can be minimized, thereby improving the effective aperture efficiency. However, a slot antenna of this type is still unsuitable for a ground antenna for transmitting and receiving microwaves in satellite broadcasting. This is because grating lobes cannot be prevented from occurring in a radiation pattern of a circularly polarized microwave from radiators formed of a number of pairs of narrow slots, which are aligned on a circular-shaped waveguide in a spiral form and each pair of which has two slots arranged in a "T" or "L" shape manner. This results in a poor directivity of the antenna. In order to eliminate the above problem using the antenna structure described in the above reference, an additional circuit (e.g., a slow-wave circuit) must be necessary, resulting in a complicated structure of the slot antenna.